Question: The scale on a map is 3cm : 10km. If the distance between two cities is 80km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 3cm on the map corresponds to an actual distance of 10km. An actual distance of 80km is the same as 8 $\cdot$ 10km. The distance between the two cities on the map is 8 $\cdot$ 3cm, or 24cm.